The present invention relates to methods for synthesis and separation of chemical compounds electrochemically, methods for generation of electrical energy, including electrochemical cells for performing such methods.
U.S. Pat. No. 4,702,804 (D. J. Mazur et al.) discloses methods for electrochemical dehalogenation of organic compounds, and particularly the destruction of halogenated aromatic compounds like polychlorinated biphenyls (PCBs) in contaminated silicon oils, transformer oils and other dielectric fluids. The methods of Mazur et al., for example, provide for treating contaminated liquids in extraction and separation zones for removing potentially harmful PCBs with an organic solvent, such as propylene carbonate, cyclopentanone, etc. The recycled dielectric fluid, substantially free of PCBs, can be returned to service while the PCBs in the organic solvent are reduced electrochemically at the cathode of an electrolytic cell. Alternatively, instead of separating the PCBs from the oil the contaminated dielectric fluid may be dispersed in the organic solvent and treated in the electrolytic cell where the PCBs are cathodically reduced to compounds of lesser toxicity, greater disposability and/or reusability.
Although Mazur et al. mention aqueous systems where the catholyte and anolyte may comprise mainly water, PCBs are not readily soluble in aqueous solution. Aqueous systems are also usually less efficient because much of the power for operating the electrochemical cell is expended in electrolysis of water, instead of breaking down the contaminant. Hence, both the catholyte and anolyte, according to Mazur et al. are--nonaqueous--which means they contain less than 5 percent by weight water.
Although the methods of U.S. Pat. No. 4,702,804 are suitable in most instances for lowering PCB levels to less than 1 ppm, it was discovered that in the electrochemical dehalogenation of aromatic compounds, and particularly in the treatment of some PCB contaminated dielectric fluids containing antioxidant/clarifiers, such as (2,6-di-t-butylp-cresol), also known as BHT, the concentration of PCBs is not always lowered to the level desired. It is believed that in a homogeneous, nonaqueous system comprising an organic solvent and an antioxidant, like BHT the antioxidant migrates to the nonaqueous solvent on the anode side of the cell where it is oxidized to a quinone compound. This newly formed compound is able to re-enter the cathode side of the cell where it competes with the PCBs for reduction sites at the cathode. The passage of such competing compounds between the anode and cathode compartments is able to occur notwithstanding the presence of a cell divider, e.g. permselective ion exchange membrane, which is suggested by Mazur et al. In addition, it is believed that biphenyl, an electrically neutral compound formed at the cathode during reduction of the PCBs, can also pass through such cell dividers and into the anolyte where in the presence of chlorine becomes rechlorinated back to toxic chlorinated biphenyls.
While the processes of U.S. Pat. No. 4,702,804 provide for important improvements in dehalogenation reactions through carbon cathodes of greater stability to electrochemical corrosion, the migration of neutral molecules between anode and cathode compartments even in the presence of a cell divider often fail to prevent unwanted side reactions from occuring at electrodes due to a lack of isolation of reactions occuring at one from those occuring at the other. This can result in poor operating efficiences. Accordingly, a principal object of the present invention relates to the discovery that a multi-phase or heterogeneous system of solvents as electrolytes in a divided cell equipped with an ion exchange membrane will restrict passage of neutral molecules between cell compartments, more effectively isolating reactions taking place at the cathode and anode thereby improving overall cell performance. Thus, one aspect of the present invention contemplates organic electro-synthesis reactions in which an electrolytic cell equipped with a permselective membrane utilizes a nonaqueous solvent at the working electrode, e.g. cathode, and an aqueous solvent at the counter electrode, e.g anode. In electro-synthesis reactions utilizing the heterogeneous electrolytes, aromatic compounds, for instance, like PCBs in a catholyte comprising a nonaqueous solvent are reduced to compounds of lesser toxicity, e.g. biphenyl and chloride ions. The biphenyl in the nonaqueous catholyte being an electrically neutral molecule is immiscible in the aqueous anolyte. The solvent interface of the heterogeneous system appears to effectively aid in retaining the neutral biphenyl and any antioxidant present in the cathode compartment with the nonaqueous solvent while an anion exchange type membrane selectively permits the negatively charged chloride ions to freely enter the anolyte compartment where chlorine is formed, mitigating possible regeneration of toxic PCBs.
Ion exchange membranes possess hydrophobic properties and inherently resist the passage of water molecules between compartments of a divided cell. In contrast, with an ion exchange membrane neutral organic molecules in a homogeneous nonaqueous system of electrolytes are able to pass more freely between cell compartments. It was discovered that transmission of such neutral organic molecules between compartments can be restricted also by employing the heterogeneous solvent system of the present invention whereby the interface of the aqueous/nonaqueous solvents provides an effective buffer to the transmission of such molecules across the membrane barrier Accordingly, the system of electrolytes comprising an aqueous solvent and a nonaqueous solvent on opposite sides of a permselective membrane of an electrolytic cell offers a broad scope of useful applications in addition to improved methods for organic electrosynthesis, etc. This includes improved methods for conducting separations in both synthesis and degradation reactions in membrane divided electrochemical cells. This may be illustrated where a neutral organic reactant in a nonaqueous solvent is electrolyzed at the working electrode to form new ionic organic molecules. Because only the ionic molecules are able to migrate through the permselective ion exchange membrane to the opposing compartment comprising the aqueous solvent, more effective separation of the ionic and neutral molecules occurs.
In addition to applications of the foregoing dual system of heterogeneous electrolytes in energy consuming cells, i.e. cells requiring the input of electrical energy for synthesis of product, the divided cell system of aqueous-nonaqueous electrolytes is also useful in energy producing cells, e.g. batteries, fuel cells, and the like. Particularly in the case of high energy-density batteries utilizing reactive metals, such as lithium, zinc, etc., where such metals are highly reactive with water. The presence of water at the active metal anode can lead to cell failure and even explosion. Hence, in a membrane divided cell the aqueous-nonaqueous heterogeneous solvent interface of the present invention more effectively restricts potentially dangerous water molecules from entering the anolyte compartment, reducing the safety hazards associated with certain novel energy producing electrochemical cells.